Raman type distributed thermal sensing (DTS) instruments have been in the commercial stream over 30-years. These systems are based on Raman scatter effects of high intensity light launched into an optical sensing fiber, in which, due to the interaction between light and glass of the fiber, part of the light energy transmitted in the optical sensing fiber is transferred. Subsequent loss of energy resulting from the transfer increases the wavelength of light (i.e., a Stokes-shift), while the transferred energy can be donated to excited state atoms to cause decreased wavelength (i.e., an Anti-Stokes shift). The amount of energy donated, and relative intensity of Anti-Stokes shifted light, is related to the amount of atoms in an excited state, which is a function of temperature. Such nonlinear shifted light can be collected and guided by the fiber to be received and analyzed.
In Raman DTS systems, light pulses are launched into an optical sensing fiber, the return time and intensity of backscattered signals are recorded and, based on the recorded time and intensity, the temperature at specific locations all along the fiber is calculated. The resultant calculation yields a fully distributed temperature sensor.
Conventional Raman DTS instruments measure slight changes in the ratio of Stokes/Anti-Stokes intensity, which are separated in wavelength approximately 100 nm apart in typical near-infrared systems. Prior to installation, the sensing fiber or cable must be calibrated for each individual sensing fiber as this coefficient, and intrinsic optical attenuation at these wavelengths will vary among different sensing fibers. Once installed, successful operation of these sensors requires isolating the fiber from hydrogen, as even small amounts of hydrogen diffused into a sensing fiber creates measurement error in which wavelength-dependent hydrogen absorption creates differential fiber attenuation (DFA) between that of the intrinsic calibrated fiber and the fiber under hydrogenated condition, and furthermore will affect the Stokes/Anti-Stokes lines differently as the wavelength separation between them is significant.
To address this problem, DTS suppliers have developed compensation methods through dual- or partial-return downhole sensing configurations and measurement protocols, which are now commonly offered as a standard feature in most DTS interrogator instruments. In a dual-ended or partial return sensing configuration, the fiber is installed into the well with fiber in a turnaround loop or sub at the end of the well, where the fiber returns all the way or partially back to the surface, respectively. With these sensing configurations, it is possible to re-calibrate DTS measurements to compensate for DFA by implementing a measurement protocol and compensation algorithm. This particular architecture has two variations, one where the sensing cable has access to both ends at the surface, and the other where the sensing cable has a “turn-around” at the bottom, and the cable travels only a portion of the way back up the well. The strategy in both cases relies on the fact that two positions of the cable (separated by a distance L) are exposed to an identical temperature. If the cable length between the two sensing points experiences a differential loss due to hydrogen diffusion into the glass or possibly from mechanical bending, there will be a temperature difference between the two points. This temperature difference is then used to compensate the particular length of fiber between the two points and correct for the temperature difference. This process is applied over the entire length of the fiber section that has overlapping temperature points and a complete picture is then stitched together to correct the measured temperature along the entire section of cable that has co-located points. The fully dual-ended system, where the cable is interrogated from each end, has the ability to compensate the entire cable from top to bottom.
While fully dual-ended methods are practiced with some success, they are difficult to implement with current well instrumentation installation procedures and equipment. Dual-ended methods also have shortcomings in reliability and performance during well operations. These challenges begin when routing or looping the fiber at the distal end, during which time the fiber, and, in particular, the turnaround loop, must be retained in a low strain condition in order to maintain mechanical reliability of the fiber. The fiber must be further retained in a low stress condition along the length of the fibers during heating and thermal cycling, such as during well operations.
Downhole cables require metal armor or tubing construction to protect the fiber and resist the well chemical and pressure environment. Conventional downhole cables typically have thick-wall ¼″ construction similar to hydraulic control lines. Unlike the latter that are hollow and transport fluid, fiber optic sensor cables house glass optical fibers that have a thermal expansion mismatch to the steel cable material, promoting significant stress on the fibers when the cable expands more dramatically upon heating. Use of excess fiber length cable design, in which a few percent-longer length of fiber is incorporated in the cable is common with downhole optical sensing cables to address this problem, using thick cable gels to retain the fiber within the cable. However, cable gels are especially difficult to impart uniformly on a fiber installed in a cable. Further, this issue is exacerbated with high temperature cables that use polyimide fiber coatings, with more difficult mechanical handling and subsequent fiber retention properties, and cannot use cable gels due to the limited upper temperature range of the cable gels. Furthermore, fiber loop and turnaround hardware, and fiber splice packaging to connect such hardware, is much larger than the cable diameter, presenting an obstruction and frequently a point of failure. Fiber turnaround devices with a small form factor relative to the cable are available based on fused-taper fiber construction, but are limited to lower temperature packaging (i.e., less than 125° C.).
Despite these difficulties, dual and partial-ended Raman DTS systems are routinely installed with spliced turnaround or looped fluid conveyance methods or other arrangements. These systems are subsequently operated using special measurement protocols and compensation algorithms that typically perform a slope correction calculated from a co-located position above the turnaround point for each fiber. While this solution presents a significant improvement in DFA errors, it is averaged, and does not account for non-uniform distribution of DFA and sources such as modal loss that is dynamic. This results in measurement error along discrete points of the sensing cable.
The object of the present invention is, therefore, to provide a cable for an optical sensing fiber, which, among other desirable attributes, significantly reduces or overcomes the above-mentioned deficiencies of prior cables.